One approach that is gaining currency among environmental scientists is carbon dioxide capture and storage (CCS), a form of carbon sequestration in which CO2 is removed from the waste gas of power plants, typically by absorbing it in a liquid, and subsequently burying it deep underground, hence keeping the gas out of the atmosphere. Abandoned oil fields and briny aquifers can impound much more carbon than the plants and soil envisioned in some sequestration schemes can, and are more practical--at least for now--than deep ocean storage.

Indeed, CCS technology is already in use. Since 1996, the Norwegian company Statoil has been stripping about a million tons of CO2 a year out of natural gas from the Sleipner West field under the North Sea and injecting it at high pressure into a saline aquifer. Avoiding Norway's tax on CO2 emissions balanced most of the costs of the project. An even larger project, begun in 2000, takes CO2 from North Dakota and sinks it into an oil field in Weyburn, Saskatchewan, Canada. Here, selling the extra oil that the injected C02 squeezes out largely offsets the costs. These efforts and others show that the sequestered CO2 can be monitored, and that it largely stays confined underground.

But these cases notwithstanding, CCS is expensive. Capturing even a modest part of the 25 billion tons of CO2 emitted each year, experts say, will require economic incentives. On December 8, the bipartisan National Commission on Energy Policy, which includes academics, industrialists and environmentalists, proposed a broad course of action for the U.S. The commission recommended a mandatory system of tradable permits to emit carbon dioxide and other greenhouse gases. (A similar mandatory system had been proposed in the McCain-Lieberman Climate Stewardship Act, which was defeated in the Senate in 2003.) To minimize economic disruption, the commission advised that the government cap the cost by selling extra permits starting at $7 per metric ton of CO2 in 2010 with a 5 percent annual increase. In contrast, in Europe, which initiated trading of carbon credits on January 1 to meet its Kyoto commitments, credits already sell for about $11 to $12 per ton.

But even the higher European credit prices aren't going to be enough to promote widespread adoption of CCS, which currently costs $40 to $100 a ton. "There's still a gap between the marketplace and the technology," says Howard Herzog of the Laboratory for Energy and the Environment at the Massachusetts Institute of Technology.

The best way to make CCS cheaper is for government to provide incentives to use it, asserts David Hawkins of the environmental group Natural Resources Defense Council. "Learning by doing is the thing that drives the cost down, not R&D dollars," he adds. Between 2000 and 2030, power plants with a lifetime potential of more than 1,000 billion tons of CO2 could be built, including many in coal-rich China, Hawkins observes. Extracting CO2 from traditional coal plants is much less efficient than from gasification plants, where coal is first turned to a gas and reacted with water to form CO2 and hydrogen. Because the new plants are somewhat more expensive and less reliable, "there's a real danger of lock-in" to plants that make CCS difficult, he warns.

Scott Klara of the Department of Energy's National Energy Technology Laboratory in Pittsburgh agrees that gasification makes CCS easier. Klara manages the DOE's carbon sequestration program, which funded $56 million of research this fiscal year, much of it in collaboration with industrial partners. The program aims to reduce the cost of CCS to $10 per ton of avoided CO2 by 2012, corresponding to a roughly 10 percent increase in the cost of electricity. He is confident, however, that even for traditional coal plants the cost of CCS can be reduced to $20 per ton.

These prices might be sufficiently low to encourage large-scale adoption of CCS. But Peter Frumhof of the Union of Concerned Scientists cautions that government investment in CCS without supporting economic incentives to reduce carbon dioxide emissions is "disingenuous at best, and dangerous at worst." Without a regulatory framework, he says, "we're not going to get there."

Even widespread adoption of CCS will not stop the accumulation of CO2 in the atmosphere. Only about a third of worldwide CO2 emissions arise from electricity generation, with much of the remainder coming from heating, industrial processes and transportation. Still, if we begin to apply a variety of CO2 reduction technologies in tandem, emissions can be held close to their current level, instead of doubling over the next 50 years, argues Stephen Pacala of Princeton University. These techniques include CCS as well as energy conservation, improved efficiency, and renewable or even nuclear energy sources. "As a species we are technologically ready to tackle the carbon and climate problems," he remarks.

Others predict that the planet's enormous geologic reserves will eventually be inadequate to hold all of the CO2 the world is likely to produce in the next hundred years. Klaus Lackner of Columbia University, for one, advocates sequestering the carbon in minerals like magnesium silicates, although currently the associated cost is much too high. Whatever the approach, "we need to do it now," he insists. "We cannot afford to sit back and say some great invention will come along sometime in the middle of the century."